The purpose of light curtains is to detect objects that enter a monitoring area. These objects can be various materials, including inanimate items, or also persons. For this purpose, a relevant area is commonly illuminated by one or several light sources, and the radiation striking one or several recipients is analyzed, wherein for example intensity differences of the radiation striking the recipients are determined, and from this inferences are made regarding the changes due to the objects that were introduced into the light path. If applicable, additional optical components can be provided, for example reflectors, lenses, filters, polarizers, as well as control and regulation electronics in general.
Consequently the presence or the entry of an object into a light barrier or a light curtain can be determined. More detailed qualitative statements regarding the objects acquired by a light barrier or a light curtain are, however, only possible with great difficulty. To some extent the external shape of an object moving through a light curtain can be identified.
An essentially point-shaped detection by means of a light ray is commonly referred to as a light beam. If an essentially line-shaped light barrier is used, a two-dimensional area therefore arises that is referred to as a light curtain, and by means of which the intrusion into an area can be verified over an extensive surface.
Light curtains are particularly used in security technology for automation applications.
In spectroscopy the wavelength-dependent absorption and emission signals of materials are investigated and analyzed. Spectroscopy in the near infrared range (NIR spectroscopy) facilitates the very exact characterization of organic chemical materials, based on rotational and vibrational bands in the absorption spectrum of the material. By these means plastics can, for example, be distinguished precisely, the contents of liquids can be identified, or also human tissue can be distinctly identified. For that purpose, the object to be investigated is commonly placed in a measurement chamber and examined using a spectrometer. WO 02/44673 A1, WO 99/40414 A1, U.S. Pat. No. 6,433,338 B1 and DE 196 01 923 C1 present methods for the examination of objects by means of spectral analysis.
US 2004 0 218 172 A1 describes the application of spatial light modulations for spectroscopy and imaging. For that purpose, a device is operated in the visible and near infrared light range. A sample can be illuminated with suitable, energy-weighted spectral bands in order to only identify relevant components. In so doing, the energy of the applied spectrally resolving elements can be modulated digitally in order to generate a tunable weighted spectral light.
U.S. Pat. No. 6,993,176 B2 describes a method and a device for the imaging of a container that is filled with a liquid, wherein light from a light-emitting device illuminates the container, and light that has penetrated the container is detected and analyzed. In this case light in the near infrared range can be emitted in addition to the visible light.
EP 0 562 726 B1 describes a method for the operation of a light curtain system with at least three light curtain segments that respectively have one transmission device for emitting a plurality of light ray bundles along channels toward receiving devices for the purpose of scanning. In this case temporal scanning is performed, wherein the channels extend at specified angles relative to one another and in a common acquisition plane in such a manner that they intersect one another. Selected ray bundles in each transmission device can be deactivated.
DE 20 2004 020 863 U1 describes a device for securing a work area that has a hazardous as well as a less hazardous or non-hazardous section. If, by means of a first security system, an intrusion of an object in the hazardous section is determined, and, by means of an additional security system, no object is determined in the less or non-hazardous section, a signal trigger level for the generation of a signal is activated.